A printing machine generally comprises a sheet transport system that receives sheets from a sheet feeder, moves the sheets through various printing stations in the printer and after the sheets are printed transports the sheets to an output tray. Sensors that “watch” for the passage of a sheet as the sheet transport system moves the sheet through the printer monitor the position of the sheet. Usually the sensors are optical sensors or contact sensors that sense a leading and/or trailing edge of the sheet as the edge passes through their respective fields of view or contract positions. The sensors do not provide continuous real time information as to where the sheet is at all times as it passes through the sheet transport system. As a result, if the sheet jams it is often difficult to locate the position at which it jammed and a position of a cause of the jam. In addition, optical and contact sensors are readily dirtied by dust, such as paper dust, from sheets transported by a transport system and have to be cleaned regularly.
U.S. Pat. No. 4,369,964 describes a sheet feed apparatus that senses if a sheet fed to an impression roller of a printer seats properly on the roller. The roller is formed with two longitudinal slots that communicate with a source of vacuum that aspirates through the slots. When a sheet to be printed is transferred to the roller, the sheet covers the slots and vacuum generated by the vacuum source at the slots as a result of aspiration of the vacuum source secures the sheet in position on the roller surface. If the sheet doesn't seat properly on the roller surface the sheet doesn't completely cover both slots. As a result, the vacuum developed by the vacuum pump at the slots drops below or doesn't attain full magnitude. A pressure sensor senses that the vacuum is below what it should be and generates an alum indicating a malfunction.
The system comprises the impression roller and a set of vacuum suckers that deliver sheets to the roller. The system does not indicate a jam or malfunction of the sucker “delivery system”. The system does not determine if a sheet is improperly seated on the roller as a result of the sheet jamming at a position of the suckers or the suckers improperly holding the sheet and/or transferring the sheet to the roller improperly.